The present invention relates to ceramic cores for use in investment casting of metallic Industrial gas turbine engine blades and vanes having internal passageways and large airfoil pitch.
In casting gas turbine engine blades and vanes using conventional equiaxed and directional solidification techniques, ceramic cores are positioned in an investment shell mold to form internal cooling passageways. During service in the gas turbine engine, cooling air is directed through the passageways to maintain blade temperature within an acceptable range. In manufacture of large gas turbine engine blades and vanes for industrial gas turbine engines, correspondingly larger ceramic cores are used to form the internal passages. The ceramic cores used in investment casting can be prone to distortion and loss of the required dimensional tolerance during core manufacture, especially of the airfoil core pitch. The problem of airfoil pitch distortion is greater for larger ceramic cores used in the manufacture of industrial gas turbine engines.
An object of the present invention is to provide a method of making a ceramic core and the core so made in a manner that reduces airfoil pitch shrinkage and loss of dimensional tolerance.
An embodiment of the present invention provides a method of making a ceramic core having an airfoil section for use in making a gas turbine engine airfoil casting by forming a precursor core (hereafter referred to as a chill) of smaller dimensions than the final desired ceramic core, firing the chill, applying a thin ceramic skin to the fired chill to form a coated core, and then firing the coated core. Firing shrinkage of the thin ceramic skin during the second firing operation is minimal compared to that of the chill in the first firing. Shrinkage, distortion and loss of dimensional tolerance of the airfoil pitch of the final core is thereby reduced.
The invention provides a ceramic core for use in making large industrial gas turbine engine airfoil castings having an airfoil pitch of one inch and greater and having an airfoil pitch shrinkage of the core of about 0.5% or less.
The above objects and advantages of the present invention will become more readily apparent, from the following detailed description taken with the following drawings.